Simulations capture life and death of a neutron

neutron (Download Image) Beta decay, the decay of a neutron (n) to a proton (p) with the emission of an electron (e) and an electron-anti-neutrino (ν). In the figure gA is depicted as the white node on the red line. The square grid indicates the lattice. Image by Evan Berkowitz/Forschungszentrum Jülich/Institut für Kernphysik /Institute for Advanced Simulation

Lawrence Livermore (LLNL) and Lawrence Berkeley national laboratory scientists, as well as scientists from UC Berkeley and other institutions, have simulated a "smidgen" of the universe to delve into determining the life and death of a neutron.

Experiments that measure the lifetime of neutrons reveal a perplexing and unresolved discrepancy. While this lifetime has been measured to a precision within 1 percent using different techniques, apparent conflicts in the measurements offer the possibility of learning about as-yet undiscovered physics.

The team of scientists, co-led by LLNL’s Pavlos Vranas, have tapped powerful supercomputers at LLNL and Oak Ridge National Laboratory (ORNL) to calculate a quantity known as the "nucleon axial coupling," or gA -- which is central to the understanding of a neutron’s lifetime -- with an unprecedented precision. Their method offers a clear path to further improvements that may help resolve the experimental discrepancy.

The finding plays a significant role in nuclear physics and the way the universe has evolved. The research appears online May 30 in the journal Nature.

The nuclei of all atomic elements are made by protons and neutrons. Protons and neutrons are not fundamental particles, rather they are made from yet smaller ingredients, the quarks and gluons. The theory that describes how quarks and gluons make protons, neutrons and all other nuclear particles as well as how they interact is called Quantum Chromodynamics (QCD).

"The proton, although it is not fundamental, is extremely stable against decay. We have never observed a proton decay and it is believed that its lifetime is longer than that of our universe," Vranas said. "This is predicted by the symmetry properties of QCD. However, the neutron, although it has similar composition and mass as the proton (it is slightly heavier than the proton), if it is left alone (if it is not part of a nucleus), it breaks apart in about 15 minutes. It decays to a proton, an electron and an anti-neutrino. This decay is called beta-decay and its rate is dictated by the axial coupling of the nucleon, gA. Therefore gA plays a central role in nuclear physics and the way our universe has evolved."

To achieve their results, the researchers created a microscopic slice of a simulated universe to provide a window into the subatomic world.

The problem is that although in principle one can use QCD to calculate gA , in practice this has proven to be a monumentally difficult task. The team used the numeric simulation known as Lattice QCD, powerful supercomputers and new techniques.

"This signals that Lattice QCD is now capable of calculating one of the important nuclear physics quantities, gA, with high precision and opens the door for a wealth of Lattice QCD calculations of importance to nuclear physics," Vranas said.

The team’s new theoretical determination of gA is based on a simulation of a tiny piece of the universe -- the size of a few neutrons in each direction. They simulated a neutron transitioning to a proton inside this tiny section of the universe to predict what happens in nature.

The model universe contains one neutron amid a sea of quark-antiquark pairs that are bustling under the surface of the apparent emptiness of free space.

"Calculating gA was supposed to be one of the simple benchmark calculations that could be used to demonstrate that lattice QCD can be utilized for basic nuclear physics research, and for precision tests that look for new physics in nuclear physics backgrounds," said André Walker-Loud, a staff scientist in Berkeley Lab’s Nuclear Science Division who led the new study. "It turned out to be an exceptionally difficult quantity to determine."

The team participating in the study developed a way to improve their calculations of gA using an unconventional approach and supercomputers at LLNL and ORNL.

Their work builds upon decades of research and computational resources by the lattice QCD community. In particular, the research team relied upon QCD data generated by the MILC Collaboration; an open source software library for lattice QCD called Chroma, developed by the USQCD collaboration, and QUDA, a highly optimized open source software library for lattice QCD calculations.

With more statistics from more powerful supercomputers, the research team hopes to drive the uncertainty margin down to about 0.3 percent.

In addition to researchers at LLNL and Berkeley Lab, the science team also included researchers from and UC Berkeley, University of North Carolina, RIKEN BNL Research Center at Brookhaven National Laboratory, the Jülich Research Center in Germany, the University of Liverpool in the U.K., the College of William & Mary, Rutgers University, the University of Washington, the University of Glasgow in the U.K., NVIDIA Corp. and Thomas Jefferson National Accelerator Facility.

The work was supported by the Lawrence Livermore National Laboratory Multiprogrammatic and Institutional Computing program through a Tier 1 Grand Challenge award, Laboratory Directed Research and Development programs at Berkeley Lab, the U.S. Department of Energy’s Office of Science, the Nuclear Physics Double Beta Decay Topical Collaboration, the DOE Early Career Award Program, the NVIDIA Corporation, the Joint Sino-German Research Projects of the German Research Foundation and National Natural Science Foundation of China, RIKEN in Japan, the Leverhulme Trust, the National Science Foundation’s Kavli Institute for Theoretical Physics, and DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program.